The Wages of Fear (Le Salaire de la peur) (1953) [Mario]: My memory of this movie is vague, but I believe he was killed when he crashed his truck going back down the mountain after safely delivering the explosives.

The Crucible (Les Sorcieres de Salem; The Witches of Salem) (1957) [John Proctor]: Presumably executed by hanging. (I haven't seen this version, but I'm reasonably familiar with the story.)

Mr. Freedom(1969) [Capitaine Formidable]: Killed (off-screen, exact method unknown) by Delphine Seyrig, shortly before the movie begins; his frozen body is shown afterwards when it arrives in a package at Freedom headquarters. (There is also a hallucination sequence earlier where John Abbey imagines seeing Yves' hanged body.)

Z(1969) [The Deputy]: Bludgeoned with a club by Marcel Bozzuffi when Marcel and Renato Salvatori drive past him after a political rally; he dies in the hospital a few days later during surgery (we only see the doctor throwing a sheet over the body).

State of Siege (Etat de siege) (1973) [Philip Michael Santore]: Killed (off-screen) by guerrillas; his body is shown sitting in a car afterwards when police discover him. (Thanks to Zoran)

Manon of the Spring (Manon des sources) (1986) [Cesar Soubeyran]: Dies of old age/natural causes in his sleep. (Thanks to Hamish)